Show guidance if the framework is not found

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 154c84a..174a99f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -277,6 +277,9 @@
     set(CMAKE_INSTALL_LIBDIR "${LIB_INSTALL_DIR}")
 endif()
 
+if (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/framework/CMakeLists.txt")
+    message(FATAL_ERROR "${CMAKE_CURRENT_SOURCE_DIR}/framework/CMakeLists.txt not found. Run `git submodule update --init` from the source tree to fetch the submodule contents.")
+endif()
 add_subdirectory(framework)
 
 add_subdirectory(include)